Various methods for manufacturing solid polymers from hydrocarbons, for example from 1-olefins have been developed. In one such method olefins, such as ethylene, propylene or butene, are polymerized in the presence of catalysts in hydrocarbon diluents or in monomers acting as diluents. The reactants are kept in liquid phase by maintaining a proper pressure in the polymerization reactor. When the polymer is insoluble or only slightly soluble in the diluent, the polymer product forms as particles suspended in the diluent and therefore the process is called a slurry process.
As batch process the above process has the advantage that all the polymer particles have the same residence time in the reactor and therefor the product quality is even. However, in commercial high production plants the polymerization reactors tend to be large. The operation is labor extensive and the quality of the products from batch to batch is not the same. For these reasons the batch reactors are not commercially acceptable.
A typical continuous slurry process is carried out in a continuous pipe reactor forming a loop, where the polymerization is carried out in a circulating turbulent flow. The product containing polymer, diluent and monomers, is taken from the loop reactor either continuously, or more usually, periodically through a discharge valve and it is introduced to a separator, where the polymer is separated by lowering the pressure.
Another reactor type in olefin polymerization art is a gas phase reactor, where polymerization is carried out in the presence of catalysts and gaseous monomers. Typically the polymerization is carried out in fluidized bed reactors, where polymerization is carried out continuouly in a bed formed by polymerizing polymer particles. This bed is kept in fluidized state by circulating gaseous flow from the top of the reactor to the bottom of the reactor. Polymerization heat is removed by cooling said circulating gaseous flow.
It is also known continuous multiphase processes, where slurry reactors, such as loop reactors are followed by one or more gas phase reactors or where two or more gas phase reactors are used in series.
A known problem in continuous processes is that even residence time of catalyst is difficult to achieve. Therefore the product quality tends to be more or less uneven. This phenomena is exaggerated in multiphase processes. The catalyst is usually fed into the first reactor only. Some of the catalyst particles react with the monomers for a longer period, whereas part of the catalysts flows straight through the reactor and will be removed more or less unreacted. In the next reactor the unreacted catalyst particles react differently with monomers and resulting, among others, in uneven product quality, gels, lumps and more difficult process control.
It is also known to prepolymerize a small amount of olefin monomer with a catalyst before using these catalysts into a main polymerization reactor. Typically such prepolymerization reduces catalyst attrition and improves the resulting polymer morphology. Prepolymerized catalysts also may suspend more readily in hydrocarbon solvents, yield polymers of higher bulk density and reduce formation of lumps in gas phase reactor.
Such prepolymerization can be carried out by contacting a solid catalyst component with a small amount of olefin monomer in a suitable diluent or monomer in a vessel separate from the main polymerization reactor.
The most convenient way to prepolymerize is a continuous prepolymerization, but due to residence time distribution, part of the catalyst will not prepolymerize enough and will produce fines in the main polymerization reactor. A batch prepolymerization often reduces the catalyst activity and there is always some difference between prepolymerized catalyst batches. Some catalysts have to be prepolymerized so much that the amount of polymer may cause the catalyst handling to be too troublesome.
Some catalyst feeders feed small batches of catalyst with a cycle time of several seconds or minutes. This sometimes causes fluctuation in prepolymerization or in actual polymerization.
The common problem in all polymerization processes mentioned above is uneven residence time distribution, which leads to uneven and undesirable product quality and more difficult process control. This problem can be to some extent avoided if tubular, very long reactors having a very small diameter is used. For example in EP 0279153 a prepolymerization method is disclosed, where the prepolymerization is carried out as a plug flow. However, that kind of reactors are difficult to control and they are not suitable for high production rates because of risk of plugging and low capacity.
Therefore need exists for olefin polymerization processes where the polymerization can be carried out so that more narrow residence time distribution can be achieved and the problems arising from the uneven polymerization degree can be avoided.